At least out to \(z\)\(\sim \) 2–3, increased gas fractions (i.e., larger gas reservoirs) have been identified as the main reason for the higher SFRs of the galaxies [1]}, [2]}. Commonly, CO lines are good tracers of the molecular gas reservoirs in galaxies. At higher redshifts, only high-\(J\)  CO transitions can be observed, which come with considerable uncertainties due to necessary excitation corrections.
At higher redshifts, the gas content of main-sequence galaxies has therefore not been studied statistically and robustly. C\(^+\)  has been shown to be a reasonable empirical predictor of the molecular gas content, which does not involve uncertainties of excitation corrections associated with high-\(J\)  CO transitions [3]}, [4]}. The relation between C\(^+\)  and gas mass is likely due to the secondary causal connection with SFR.
With ALPINE data, the first statistical assessment of gas masses and fractions can be undertaken via their estimation from C\(^+\) . Together with a quantification of the contribution of mergers (derived from close-pair counts in sub-mm data) a full picture of mass growth in \(z=\)  4–6 galaxies can be obtained.
